[[WMG: The creatures on the Undead cards look nothing like the actual Undead...]]
...because several cycles ago, the battling Undead grew so powerful that they caused a global cataclysm, and the winner back then, Human Undead, demanded that all Undeads' powers be nerfed down till they were all humanoid-sized. It's the very reason none of the Undead animals are dinosaurs.
** Jossed, we had a flashback of the original battle, and they were the same.

[[WMG: The reason that all of the undead are humanoid, is because the human undead won the previous battle.]]
The undead take on a form roughly similar to the current dominant species on earth. If, for example, the stag beetle undead had won, we would be watching a bunch of beetle-monsters duking it out instead. Also, as with the above theory, the images depicted on the undead cards are likely their true forms.
** Jossed, they were humanoid during the battle fight original.

[[WMG: [[Manga/CardcaptorSakura The Clow Cards]] were based on the entire undead cards except for Joker]]
All because the creator does not like fugly animals, he revised his version with mostly BishounenLine elements and beat plagerism in the process!